The present invention relates to a motor using a permanent magnet as in a spindle motor.
Spindle motors include a permanent magnet and an armature each having a plurality of pole surfaces such that the pole surfaces are provided in a relationship of being in a direction of rotor rotation. In the motor of this kind, the permanent magnet is formed by magnetizing a cylindrical or ring formed permanent magnet material in a rotor radial direction such that N-poles and S-poles exist alternately with respect to a rotation direction.
In a motor desired for size reduction, particularly in thickness dimension (dimension in rotation axis direction) as in a spindle motor for a hard disk drive (HDD), a cylindrical or ring formed permanent magnet is arranged as one member on the rotor, with an armature as one member arranged on an inner side of the permanent magnet on the stator.
The motor of this kind switches electrically or mechanically the direction of a current flowing to the armature to periodically switch the direction of a magnetic field to be caused by the armature, thereby acting separately Lorentz""s forces upon the permanent magnet or armature and obtaining a rotation force.
However, in the motor of this kind, the pole surface edges (or magnetic pole boundary) of the permanent magnet and armature in a rotor rotation direction extend in a straight line form and parallel with a rotation axis. This however provides a structure that the magnetic field formed by the pole surface of the permanent magnet and the pole surface of the armature rapidly changes in their space distributions. As a result, a so-called cogging phenomenon occurs so that the rotor will not rotate smoothly.
Therefore, it is an object of the present invention to suppress the cogging phenomenon from occurring in a motor, such as a spindle motor.
A motor according to the invention includes a permanent magnet arranged on one of a rotor and a stator, and an armature arranged on the other of the rotor and the stator. The permanent magnet is magnetized such that opposed areas of pole surfaces of the permanent magnet and the armature gradually increase when the pole surface of the permanent magnet enters a magnetic field region of the pole surface of the armature or gradually decrease when the pole surface of the permanent magnet leaves a magnetic field region of the pole surface of the armature.
As a result of various studies by the present inventors, it was revealed that cogging phenomenon occurs due to rapid change of a magnetic field distribution formed by the permanent magnet and armature when the permanent magnet at its pole surface enters and leaves a magnetic field region of the pole surface of the armature.
Accordingly, when the permanent magnet at its pole surface enters a magnetic field region of the pole surface of the armature, if the pole surface opposed areas of the permanent magnet and armature gradually increase, the change rate of a magnetic field distribution formed by the permanent magnet and armature decreases, resulting in less possibility of causing cogging phenomenon. Also, when the permanent magnet at its pole surface leaves a magnetic field region of the pole surface of the armature, if the pole surface opposed areas of the permanent magnet and armature gradually decrease, the change rate of a magnetic field distribution formed by the permanent magnet and armature decreases, resulting in less possibility of causing cogging phenomenon. Therefore, the present invention suppresses cogging phenomenon from occurring.
The permanent magnet may have a cylindrical form having the pole surface in an inner peripheral surface thereof, and the armature can be arranged on an inner side of the permanent magnet. This can reduce the size, particularly thickness dimension (dimension in the rotation axis direction), of the motor, as in the spindle motor for HDD.
The permanent magnet may be magnetized such that the opposed areas of pole surfaces of the permanent magnet and the armature gradually increase and gradually decrease when the pole surface of the permanent magnet enters a magnetic field region of the pole surface of the armature and leaves a magnetic field region of the pole surface of the armature, respectively.
However, the permanent magnet may be magnetized such that the opposed areas of pole surfaces of the permanent magnet and the armature gradually decrease when the pole surface of the permanent magnet leaves a magnetic field region of the pole surface of the armature. Also, the permanent magnet may be magnetized such that the change rate of the opposed areas of pole surfaces of the permanent magnet and the armature gradually increase when the pole surface of the permanent magnet enters a magnetic field region of the pole surface of the armature.